Git commit hash support, signed artifacts publishing, and releases to sonatype

Hey guys,

I'm sending this request for you to pull the following changes:
- git commit SHA included in the version (IE: 0.0.15-32416546987684)
- ability to publish signed versions of the artifacts (through sbt-pgp plugin)
- publish releases to sonatype, through "release" command (sbt-release plugin) and then performing "sonatypeRelease" in order to close & promote the staging (sbt-sonatype plugins)